Problem Solving

If you are looking at a tuft of white or greyish cotton-wool growth on a fish, you are almost certainly looking at Saprolegnia, the water mould hobbyists just call fungus. The first thing worth saying is that it rarely attacks a healthy fish out of nowhere. Fungus is an opportunist. It settles on tissue that is already damaged, dead or weakened, which means the fluffy growth is a symptom of an earlier problem as much as a disease in its own right. Treat only the fuzz and it tends to come back; find out why the fish was vulnerable and you usually stop it for good. I have dealt with it a fair few times over the years, most often on a fish that had been nipped, chilled or kept in water I had let slide, and the pattern is always the same. ...

If your shell-dwellers have settled in, claimed their shells and still show no sign of fry, the usual cause is not something exotic. In nearly every stalled colony I have been asked about, it comes down to one of five things: the fish are too young, you have the wrong mix of sexes, there are not enough shells, the sand is too shallow, or something is stressing them. The good news for anyone on hard tap water is that the one variable people panic about most, water chemistry, is almost certainly already in your favour. Shell-dwellers come from Lake Tanganyika, and that lake is harder and more alkaline than most of our taps. My multies spawn in water straight from the cold line at about 22 to 23 dGH, and they could not care less. ...

If your tank is a few weeks old and a brown, dusty film is creeping over the glass, the sand and the leaves, take a breath. That is almost certainly a diatom bloom, and it is one of the most normal things a young aquarium does. Nearly every tank I have set up has gone through it. The good news is that it usually clears itself as the tank matures, the cleanup is easy, and you do not need to panic-buy anything. Let me show you how to be sure that is what you are looking at, why it happens, and what actually shortens it. ...

If your test kit shows ammonia, act today, not at the weekend. The single most effective thing you can do is a big water change with clean, dechlorinated, temperature-matched water, assuming your tap reads lower than your tank. Everything else here is about buying time and stopping it happening again. And there is one detail the general guides skip that matters a great deal on water like mine: hard, alkaline tap makes the same ammonia reading far more toxic than it would be in soft, acidic water. If you keep fish on hard water, read the pH section below before you do anything else. ...

Nitrate is the last stop on the nitrogen cycle, the thing that builds up in the water once your filter is doing its job properly. It will not kill a fish overnight the way ammonia does, so it is easy to ignore, and that is exactly the trap. Let it climb and sit high for weeks and you get stunted growth, dull colour, more algae than you can scrub off, and fry or shrimp that quietly refuse to do well. The fastest way to bring it down is a water change, as long as your tap water holds less nitrate than your tank does. Everything else on this page is really about slowing how quickly it climbs back up again. ...

Few things worry a new fishkeeper more than watching a fish that cannot stay level. It bobs up to the surface and cannot swim down, or it lies on its side on the substrate, or it swims nose-down in slow circles. Nine times out of ten what you are looking at is a swim bladder problem, and the good news is that the most common cause is something you can fix at home and, better still, prevent. I have watched a fair few of my own fish go through this, mostly greedy livebearers that ate too much, and the pattern is usually the same. ...

If your tap water is hard, you will meet limescale sooner or later. It shows up as a chalky white line where the water sits, a foggy film on the lid, and a crust creeping up the heater and the filter outlet. The first thing worth saying is that it will not hurt your fish. Limescale is a cosmetic nuisance, not a health problem, and once you understand what it is you can stop panicking about it and start managing it. I keep tanks on tap water that runs about 22 dGH, so I have scraped a fair amount of the stuff off glass over the years, and it turns out to be one of the easier problems in the hobby to live with. ...

If you keep African cichlids for long enough, one day a fish that was fine last week will be sulking in a corner, refusing food, and trailing a length of clear or white stringy waste. That is the start of what most keepers call Malawi bloat, and it is the single problem that has cost me the most fish over the years. It moves faster than you expect, it does not always respond to treatment, and the advice online is a mess of half-truths. The most useful thing I can tell you up front is that bloat is very rarely about your water being too hard. On my tap, which runs about 22 dGH and alkaline, Rift Lake cichlids are exactly where they want to be. What tips them into bloat is almost always what goes in their mouths, and how stressed they are while it happens. ...

There is no salt in any of my freshwater tanks, and there has not been for years. That is the short version, and for a healthy community tank it is also the complete version. Aquarium salt has a reputation as a general tonic, a splash of it in every tank to keep the fish perky, and that reputation is mostly leftover folklore from decades ago. It does have one genuinely useful trick, and a couple of narrow treatment roles, but the “add a bit to be safe” habit does more harm than good. Let me untangle what it actually does from what it is claimed to do. ...

Ragged fins are one of the first things a new keeper panics about, and fin rot is usually the diagnosis they land on, sometimes rightly. I have dealt with it more times than I would like, mostly in my early years when my water changes were casual and my tanks were more crowded than they should have been. The good news is that fin rot is rarely a death sentence and almost never a mystery. It follows poor conditions the way damp follows a leaking roof, and once you understand that, the fix stops being about chasing a magic bottle and starts being about the water. This is how I recognise it, treat it, and keep it from coming back. ...